The present invention relates to optical data storage devices. More specifically, the present invention relates the control and operation of the laser used in optical data storage systems to produce a low noise optical signal for use in reading data from the optical media.
Since the launch in 1982 of the audio CD, optical disks have become a very popular storage media due to their durability, random access features, and the high capacities that can be achieved on a single removable disk. The computerization of businesses has also steadily increased the amount of data that is processed. As more data is processed, the amount of data which must be stored increases as well. To meet the need of this ever-increasing amount of data, cost-effective data storage is desired. To remain competitive and to meet the needs for storage, increasing the disk capacity is a paramount development goal for optical drive products. (See, P. Asthana, B. I. Finkelstein, and A. A. Fennema, “Rewritable optical disk drive technology,” IBM Journal of Research and Development, Vol. 40, No. 5 (1996)).
As is well known, data on optical disks is stored by altering the physical properties of the optical media at defined locations. The optical media can be altered either at a factory where storage media is mass-produced, or, within the optical drive itself. Most often, the optical media is in the form of an optical disk.
For both reading and writing of information, the optical storage device utilizes a laser positioned adjacent the storage media. In the write mode, the laser is used to alter the physical properties of the media at the desired locations. Conversely, in the read mode the laser is used to illuminate the storage media surface and detect the physical surface property at the desired location. The operation and performance of the laser in both the read and write modes is critical to the efficient operation of the storage device.
As suggested above, the storage capacity of storage devices is a continuing concern. It is always desirable to store more information on a single device, rather than having to spread this across multiple devices.
One way to achieve higher storage capacity is to increase the density of information. In the case of optical storage devices, this is achieved by placing data points closer to one another on the optical media. In order to create a useful storage device however, it is critical to be able to differentiate between the various data points. Thus, data points can potentially be placed too close together, resulting in meaningless information.
The optimum placement of data points on the optical media is largely controlled by the type of laser used, and its related focal point. Obviously, the optics and lasers have limits which affect the density achievable on the storage media. If the density is excessively increased, the integrity of the data will be compromised. Again, this results in unusable, inefficient data storage devices.
Newer lasers and compatible storage media have recently made higher density data storage possible. For example, violet lasers (approximately 405 nm wavelength) have a much sharper focal point and thus allow for more high density storage. However, these new lasers appear to generate more noise than previous lasers (e.g., red lasers operating at approximately 650 nm wavelength). With the newer lasers and tighter data density, noise in the laser itself can compromise the system's ability to differentiate between data signals and noise. Consequently, it is desirable to provide a very clean laser beam, free of any significant noise, in order to make higher density optical storage possible.
Commercially available laser driver circuits provide one mechanism to control the laser itself. Specifically, the laser drive is typically used as part of a close loop low frequency (LF) control system to provide appropriate continuous wave (CW) laser-power to the media. This type of control is specifically used to control the laser during read operations. In these systems, part of the collimated laser beam is diverted to a photo detector which monitors the laser's output. The output from this photo detector is then fed back to a digital signal processor or to an analog control loop, for laser read power control. In turn, the digital signal processor provides signals to the laser driver to appropriately adjust the read power.
One function of the laser driver is to provide a controlled current source to drive the laser. The supplied current is dependent upon various signals provided to the laser driver. The laser driver itself includes some internal noise which results in noise on it's output signal. Additionally, the laser itself is also known to generate some noise as well. Consequently, any effort to control and minimize the noise created by these two sources is a beneficial improvement.
The noise problems outlined above is further exaggerated by the typical operating environment of an optical storage system. More specifically, optical storage devices typically include RF modulation in order to keep the lasers operating in a stable and quiet mode. Generally speaking, noise in the laser is minimized by utilizing higher amplitude RF modulation signals. However, use of these high amplitude RF signals is often prohibited due to RF emission requirements. Consequently, lasers are typically operated with non-optimal RF levels, which will create an additional noise problem with the new violet lasers. This simply highlights the benefits of laser noise reduction at virtually all levels.